Gravitational wave (GW) detection promises to open an exciting new observational frontier in astronomy and cosmology. In contrast to light, gravitational waves are generated by moving masses - rather than electric charges - which means that they can tell us about objects that are difficult to observe optically. For example, binary black hole systems (which might not emit much light) can be an ample source of gravitational radiation. In addition to providing insights into astrophysics, observations of such extreme systems test general relativity and might influence our understanding of gravity. Cosmologically, since GWs are poorly screened by concentrations of matter and charge, they can see places other telescopes cannot - even to the earliest times in the universe, beyond the surface of last scattering.